Add Sec-Websocket-Extension header

[akonradi-signal]

[seanmonstar]

My initial feeling with sufficiently more complex headers like this one is to let it be local to your own crate, at least to start with. It should be possible to impl headers::Header for local::SecWebSocketExt {}.

[akonradi-signal]

It's possible, and that's where I started, but then ended up with duplicate versions of private utilities in this crate, like the code in the csv module.